This invention relates to a throw-away insert, and more specifically to an improvement in a chip breaker which will facilitate smooth chip disposal during operation between a medium cutting operation and a heavy cutting operation.
In cutting operations, it is extremely important to break and discharge the chips smoothly for better workability. To achieve this, various chip breakers have been developed. It has been a common practice to consider three cutting operations, i.e. light cutting, medium cutting and heavy cutting operations dependent upon the cutting conditions (cutting speed, feed rate, and depth of cut) as shown in FIG. 4, and to develop a chip breaker suitable for carrying out each cutting operation. But because a lower depth of cut can now be made as a result of improvements in the dimensional accuracy and increased feed rate under which the material can be machined, new cutting operations, which are not covered by any of the above-said cutting operations, tend to become required.
These new operations are two which, as represented by the new high-feed rate region shown in FIG. 4, do not overlap with the medium cutting operation o the heavy cutting operation.
The more important among the two operations is the one performed at a higher feed rate than the medium cutting operation and carrying out a smaller depth of cut than the high cutting operation, that is, the hatched region in FIG. 4. This is because the chips produced in carrying out the operation corresponding to this region tend to be thicker and more difficult to break. Thus the development of a chip breaker which is effective for carrying out this operation is desired.
Prior art throw-away inserts disclosed in Japanese Examined Utility Model Publications 57-30004 and 1-15442 can be used to carry out both the light cutting operation and the medium one. But these inserts have the following problems.
When the depth of cut is so small as to be less than or equal to the nose radius, the chips are discharged substantially in the direction of the bisector of the nose angle. It is well known that as the depth of cut increases, the direction in which the chips are discharged approaches the direction in which the straight portion of the cutting edge extends (opposite to the feed direction).
For the new high-feed rate region where the depth of cut is 1-5 mm and the feed rate is 0.3-0.7 mm/rev., an insert having a nose radius of 1.2-1.6 mm is ordinarily used. In such a case, as the depth of cut increases, the chip discharge angle .theta. will change from the state shown in FIG. 5 where the depth of cut is small (1-2 mm) to the state shown in FIG. 6 where it is large (4-5 mm).
For a medium feed rate (f=0.314 0.4 mm/rev.), a narrower chip breaker as shown in FIG. 7 is preferable, whereas for a high feed rate, a wider chip breaker as shown in FIG. 8 is effective because the chips produced in such a state tend to be thick and difficult to break.
Since, as described above, the direction in which the chips are discharged changes with the depth of cut and the optimum width of chip breaker changes with the feed rate, it was very difficult to design a chip breaker which could be used not only to carry out the conventional cutting operation but also the new operations in which the depth of cut is to be small and the feed rate is to be high.
The throw-away inserts disclosed in the above-mentioned publications both define a breaker groove near each corner thereof and a protrusion extending toward the tip of the corner to in effect expand the cutting operating range. But because such a protrusion narrows the width of the chip breaker groove, it tends to cause chip clogging at a high feed rate. Thus, with these inserts, the cutting operating range can be expanded from the low cutting operation toward the medium one to some extent. But they cannot be used to carry out a heavy cutting operation in which the feed rate is high.